Stage-specific embryonic antigen 4 (SSEA4) is a hexasaccharide belonged to globo-series glycosphingolipids (GSLs) and comprises the structure of Neu5Acα2→3 Galβ1→3 GalNAcβ1→3 Galα1→4Galβ1→4Glcβ1. Since SSEA4 was first isolated from human teratocarcinoma cells in 1983 (Kannagi R, et al., 1983), it is widely used as a surface marker to define human embryonic stem cells (hESCs) so far. In past decades, more and more studies indicated that GloboH, a GSL which shares the core structure Galβ1→3GalNAcβ1→3Galα1→4Galβ1→4Glcβ1 (SSEA3) with SSEA4, is overexpressed in many epithelial cancers, including ovarian, gastric, prostate, lung, breast, and pancreatic cancers (Zhang S, et al., 1997). And high-level expression of SSEA4 was observed in renal cell carcinoma (Saito S, et al., 1997) and glioblastoma multiforme (Lou Y W, et al., 2014). More interestingly, together with SSEA3 and GloboH, the expression of SSEA4 was found not only in breast tumor cells but also in breast cancer stem cells (Chang W W, et al., 2008; Huang Y L, et al., 2013).
Carbohydrate antigens, however, are often tolerated by the immune system and consequently induce weak or non-specific immune response (Stein K E, et al., 1992; Snapper C M, et al. 1996.). It is proposed that the carbohydrate antigens are unable to be internalized and digested by the antigen presenting cells (APC), such as macrophages, B cells or dendritic cells, and therefore cannot be presented to helper T (Th) cells. The lack of simulations from APC to T-cell results in the absent of antibody maturation and isotype switching. Accordingly, low affinity and non-class-switching IgM antibody against carbohydrate antigen is predominately produced (Musher D M, et al. 1990; Lortan J E, et al. 1993). Various approaches have been developed to address the deficiencies. Conjugating carbohydrate antigens with carrier proteins to improve the immunogenicity has been developed since 1950s (Lindberg A A, et al., 1999). Such kind of highly immunogenic proteins include diphtheria toxoid (DT), tetanus toxoid (TT), CRM197 (a non-toxic variant of diphtheria toxin), and a complex outer-membrane protein (OMP) mixture from N. meningitides (Ada G. et al., 1999). In addition to the intrinsic immunogenic property of these proteins, a booster effect is expected if the recipient had been immunized with these toxoids before. The carrier protein-carbohydrate antigen conjugates provide peptides conjugated with certain carbohydrate antigen to be processed and presented by APC through MHC II molecules. With the co-simulation from Th cells, T and B cells against certain carbohydrate antigen are then activated. Followed by antibody isotype-switching and maturation, the IgG antibody against certain carbohydrate antigen with high affinity and specificity could be further generated (Bazendale H E, et al., 2000). WO 2016029071 provides a carbohydrate based vaccine comprising synthetic SSEA4 analogs chemically conjugated to the immunogenic carrier diphtheria toxin cross-reacting material 197 (CRM 197) via a linker.
Although the carrier proteins in the carbohydrate vaccination provide a solution to improve the immunogenicity, the strategy poses some new and existing problems (Ingale S, et al., 2007). First, the foreign carrier protein and the attaching linker may elicit strong immune responses, thereby leading to the suppression of an antibody response against the carbohydrate antigen. Second, the chemical conjugation is basically on the lysine of the protein surface. The experiment process is difficult to control, resulting in the heterogeneous composition and final structure. The ambiguous composition probably causes different immune response. Third, the conjugation to mimic the expression of the carbohydrate on the cell surface is not ideal, thereby the induced antibody somehow is failed to recognize the carbohydrate cluster. Alternative approaches, such as carbohydrate PEGylation (Giorgi M E. et al., 2014), are investigated to overcome the remaining problems.
Nevertheless, the active immunization therapy mentioned above is not practice well in cancer patients who is in the status of hypoimmune. Particular those who receive chemotherapy or radiation therapy, as well as late-stage cancer patients, the efficacy of active immune intervention is often limited.
In view of the foregoing, instead of the vaccination, there exists a need to develop a therapeutic antibody against the cancer carbohydrate epitope to adapt passive immunity.